Refrigeration systems normally comprise one or more compressors, one or more condensers and one or more refrigeration entities arranged in a refrigerant path. Each refrigeration entity comprises one or more evaporators and an expansion device arranged to provide a flow of refrigerant to the one or more evaporators. The evaporator(s) provide(s) refrigeration to a refrigerated volume, and the temperature inside the refrigerated volume can be controlled by allowing or preventing a refrigerant flow through the evaporator(s).
The refrigeration entities may, e.g., be cooling and/or freezing cabinets, e.g. of the kind applied in supermarkets.
In such refrigeration systems, the cooling capacity provided by the compressor(s) is normally adjusted in order to match the refrigeration load of the refrigeration entities. This may, e.g., be obtained by switching compressors on and off. However, switching a compressor repeatedly on and off results in wear on the compressor. It is therefore desirable to minimise the number of times the compressors are switched on and off.
EP 1 980 805 discloses a method for controlling a refrigeration system. The suction pressure is controlled to be within a predetermined suction pressure range by permitting or preventing flow of refrigerant into the evaporator of one or more refrigeration entities of the refrigeration system. The compressors are only switched on or off if the compressor capacity does not match a desired capacity level. In the method of EP 1 980 805 the control of the refrigeration entities is performed centrally. This will normally require a central control system, as well as appropriate programming of the central control system.
U.S. Pat. No. 5,867,995 discloses providing a suitable pressure to the evaporators. A distributed control unit, DCU, reports a required suction pressure to a compressor controller, which adjusts the compressors accordingly. In this manner, sub-loops of refrigeration cases achieve maximum efficiency while the compressors run at the highest possible suction pressure. Operation of the DCU includes, that when the refrigeration system is operating, each DCU periodically queries case controllers for information about the performance of corresponding electronic expansion valves, EEV. This information includes flow rate of the EEV, relationship of case temperature to a temperature set point, and relationship of a temperature difference, TD, set point to maximum and minimum TD values. If the DCU instructs the case controllers to change the EEV set points, the DCU informs the compressor controller of the new suction pressure requirement, as determined by a new saturated suction temperature, SST. If the new suction pressure requirement is lower than that currently provided by a compressor rack, the compressor controller alters the compressor settings to produce the reduced suction pressure requirement. If the new requirement is not lower than the current setting, the compressor controller takes no action. The DCU then continues to monitor EEV operation.
Providing a suitable pressure to the evaporators only affects the compressors in that it optimizes the suction pressure, which the compressor is designed to stabilize. Thus, the only effect on the compressor control is that the set point for the suction is influenced by the evaporator controllers. U.S. Pat. No. 5,867,995 does not disclose adjusting a refrigeration load, that is load of the refrigeration cases, to correspond to the cooling capacity of the compressors, for thereby reducing the number of starts and stops of compressors. U.S. Pat. No. 5,867,995 discloses the opposite, that is, adjusting the cooling capacity of the compressors to the refrigeration load.